1. Field of the Invention
The present invention relates to the improved preparation of certain catalysts, and, more especially, to the improved preparation of catalysts based upon the oxides of molybdenum and/or tungsten.
2. Description of the Prior Art
Catalysts based upon the oxides of molybdenum and/or tungsten are per se known to the prior art. Same are useful, in particular, for the preparation of .alpha.,.beta.-unsaturated aldehydes by the oxidation of olefins in gaseous phase.
Thus, French Pat. No. 1,514,167 features certain catalysts for the preparation of unsaturated aldehydes via the air or oxygen oxidation of olefins, such as propylene and isopropylene, such catalysts having the following general formula: EQU Ni.sub.a Co.sub.b Fe.sub.c Bi.sub.d As.sub.e P.sub.f Mo.sub.g O.sub.h
wherein a ranges from between 0 and 20, b between 0 and 20 (a+b being between 0.5 and 20), c between 0.5 and 8, d between 0.1 and 7, e between 0 and 3, with f being less than 0.1, g equal to approximately 12, and h being between 36 and 98.
These catalysts are prepared by adding an aqueous solution of suitable water soluble salts of nickel, cobalt, iron and bismuth, and a suitable compound of arsenic and of phosphorus, to an aqueous solution of a suitable molybdate, such as ammonium molybdate. The resultant slurry is heated, with a support if so desired, to eliminate the water and to dry the solid cake that is formed. The solid cake is then calcined in air, at an elevated temperature. The "suitable" water soluble salts noted in the aforementioned patent are nickel nitrate, cobalt nitrate, ferric nitrate and bismuth nitrate, for example.
French Pat. No. 1,604,942 relates to the preparation of acrolein, which consists of oxidizing propylene in gaseous phase, with a gas containing oxygen and water vapor, in the presence of a catalyst based upon the oxides of molybdenum, bismuth, iron and cobalt, and having the following elemental composition: Mo, 40 to 67.7%; Bi, 1.9 to 21.7%; Fe, 1.6 to 6.5% and Co, 21.0 to 48.1%.
The process for the preparation of these catalysts described in the above-cited patent consists of adding, to a vigorously agitated aqueous solution of ammonium molybdate, an aqueous solution of iron, cobalt and bismuth nitrate. Subsequently, the water is eliminated and the resultant product is calcined, optionally on a support.
Another prior art reference, French Patent Application No. 72/20810, published under No. 2,147,933, features a process for the preparation of unsaturated compounds having carbonyl functions from olefins. According to this particular process, the catalytic oxidation is effected in the presence of a catalytic oxide, wherein the atomic proportions of the elemental components Co/Fe/Bi/W/Mo/Si/Ti/Z (Z=alkali or alkaline earth metal) are 2.0 to 20.0/0.1 to 10.0/0.1 to 10.0/0.5 to 10.0/2.0 to 11.5/0.5 to 15.0/0.005 to 3.0/0 to 3.0, with the proviso that W+Mo is equal to 12.0.
And these particular catalysts may be prepared by mixing aqueous solutions of ammonium molybdate and of ammonium p-tungstate, by adding to the mixture aqueous solutions of cobalt nitrate, ferric nitrate, bismuth nitrate, thallium nitrate, together with the aqueous solution of a hydroxide or nitrate of an alkali or alkaline earth metal. Subsequently, the water is evaporated and the product obtained is calcined, optionally on a support.
There is also known to the prior art, in French Patent Application No. 76/27531, published under No. 2,364,061, a catalyst based upon the oxides of cobalt, molybdenum, bismuth and iron, having the formula: Co.sub.a Mo.sub.12 Fe.sub.b Bi.sub.c O.sub.x, with a being between 8 and 10, b between 0.5 and 2, c between 0.5 and 2 and x satisfying the valences, wherein the active phase contains a phase corresponding to the formula Bi.sub.2 Mo.sub.2 Fe.sub.2 O.sub.12. These catalysts make it possible to obtain, in very appreciably improved yields, .alpha.,.beta.-unsaturated aldehydes, for example, by oxidation of olefins.
These catalysts are prepared by reacting a solution of ammonium heptamolybdate and a solution of the nitrates of Co, Bi and Fe. The water is then evaporated and the resulting paste is dried. The solid obtained is subjected to a potential precalcination at approximately 450.degree. C., a first calcination at a temperature between 450.degree. C. and 500.degree. C. for at least 5 hours, and then, after cooling in ambient air, to a second calcination under the same conditions as the first.
It will thus be appreciated that there exist a considerable number of references in the prior art describing the employment, in the oxidation of olefins (and particularly the oxidation of propylene to acrolein), of catalysts based principally upon the oxides of cobalt, iron, bismuth and molybdenum. A certain number of other oxides of metals too have been proposed as adjuvants to the base composition. It will also be appreciated that all of the processes for the preparation of these catalysts employ, in the first stage thereof, the reaction between ammonium heptamolybdate and the nitrates of iron, cobalt and bismuth.
During this reaction, ammonium nitrate is formed, which decomposes thermally at approximately 220.degree. C. during subsequent calcination.
It is thus obvious that this thermal decomposition of ammonium nitrate poses serious practical and technical problems. In fact, those skilled in this art are very well aware that ammonium nitrate is an explosive compound and that, therefore, its handling on an industrial scale is quite dangerous.
In certain other processes there are also used, in place of the nitrates, certain metal chlorides, which enter the composition of the catalyst desired. In one such process, ammonium chloride, NH.sub.4 Cl, is formed, which, during thermal decomposition, sublimates at about 340.degree. C. and is deposited in the form of a white powder in the process apparatus. The resulting clogging is obviously harmful during catalyst manufacturing operations.
It should also be emphasized that, in industrial practice, it is extremely difficult, if not impossible, to eliminate the simultaneous presence of ammonium salts and of ammonium salts and nitrates, and of chlorides.
Thus, serious need exists in the prior art for a process for the preparation of catalysts based, in particular, upon the oxides of molybdenum and/or tungsten, employing the reaction of at least one ammonium salt and at least one chloride and/or nitrate of the aforesaid metals, but wherein there is no danger, during the subsequent calcinations, of the thermal decomposition of ammonium nitrate and/or the sublimation of ammonium chloride.